Return to Contents  Chapter 9 : Alkynes  Ch 9 contents

Functional group suffix = -yne (review)
Disubstituted alkynes, R-C≡C-R', are described as "internal" alkynes because the C≡C unit is "inside" the structure.
Monosubstituted alkynes, R-C≡C-H, and the unsubstituted alkyne (ethyne) H-C≡C-H are described as "terminal" alkynes because the C≡C unit at the end of the structure. 

ethyne (or acetylene)


Classify each of the following as an internal or a terminal alkyne:
(a) 1-hexyne  ANSWER (b) 3-octyne  ANSWER
(c) cyclooctyne  ANSWER (d) propyne  ANSWER


Rank 1-hexyne and 3-hexyne for each of the following properties:
(a) heat of hydrogenation  ANSWER   (c) heat of formation  ANSWER
(b) heat of combustion ANSWER   (d) stability ANSWER


  • The alkyne functional group consists of two sp hybridised C atoms bonded to each other via one σ and two π bonds.
  • The 2 π bonds are produced by the side-to-side overlap of the two pairs of p-orbitals on the C atoms not utilised in the hybrid set.
  • The substituents are attached to the C≡C via σ bonds.
  • The 2 C of the C≡C and the 2 atoms attached directly to the C≡C are linear.
  • Since alkynes are linear, they cannot exist as cis- / trans- isomers.
pi(1) bond molecular orbital of ethyne pi(2) bond molecular orbital of ethyne
pi bond molecular orbitals of ethyne
The two separate perpendicular π molecular orbitals
Combined π molecular orbitals

Physical Properties:

As with hydrocarbons in general, alkynes are non-polar and are insoluble in water but soluble in non-polar organic solvents.


  • The π bonds are a region of high electron density (red) so alkynes are typically nucleophiles.
  • Alkynes typically undergo addition reactions in which one or both of the π-bonds are converted to new σ bonds.
  • Terminal alkynes, R-C≡C-H, are quite acidic (indicated by blue) for hydrocarbons, pKa = 26
  • Deprotonation of a terminal acetylene gives an acetylide ion.
  • The acetylide ion is a good nucleophile and can be alkylated to give higher alkynes.
electrostatic potential of ethyne

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary